Almost all systems which utilize light beams for communications, measurement, experiment or otherwise, include devices for detecting the light. Such devices are generally referred to as optical detectors which, in most communications or measurement applications, comprise semiconductor photodiodes.
A semiconductor photodiode comprises a semiconductor surface which, when exposed to light, causes an electrical current to flow on the surface. The output of a semiconductor photodiode type optical detector is typically coupled to processing electronics which amplify and condition the produced electrical signal. For example, circuitry may be provided 1) to sharpen the edges of light pulses by detecting when the signal strength passes a threshold level and cutting off the signal at that point or 2) to check and modify the timing of a series of pulses.
How well an optical detector performs its function of detecting light and producing electric current therefrom is sometimes termed quantum efficiency and depends on 1) its sensitivity to light striking its detection surface, 2) the intensity of the light which strikes the detection surface and 3) the reflectivity of the detector surface.
Sensitivity of a semiconductor photodiode depends on the operating conditions, the detector design, and how the particular semiconductor material used responds to light of different wavelengths. For instance, silicon photodiodes are particularly sensitive to light of 0.8 to 0.9 micrometer wavelength but are not usable for detecting light of 1.3 micrometers in wavelength. Other semiconductor photodiode materials such as germanium or indium gallium arsenide (InGaAs) are more sensitive to longer wavelengths.
Normally, significant light energy is wasted at the optical detector because a percentage of the light which strikes the photodiode is reflected off of the semiconductor surface instead of absorbed. It is not unusual for 20% or more of the light which strikes the surface of a semiconductor photodiode type optical detector to be reflected and wasted. The less light which is reflected and wasted, the higher the quantum efficiency of the photodiode.
U.S. Pat. No. 3,815,977 issued to Vasiliev discloses a photo detector incorporating a prism above the detecting surface for returning light which was reflected off of the detector surface back to the detector surface. Vasiliev teaches two different embodiments. In the first embodiment, incident light reflected off of the photo-detector surface strikes a face of the prism and is returned to a different point on the detector surface. Reflection off of the detector surface from that second point is again reflected off of a different face of the prism and returned to a third point and so on for subsequent reflections. In the second embodiment taught by Vasiliev et al., light is returned to the original point of incidence on subsequent reflections, however, the light is reflected off of two separate surfaces of the prism before being returned to the point of incidence. As in the first embodiment, light is returned to the detector surface a plurality of times, but with each return requiring reflections off of two separate surfaces of the prism rather than one. The photo-detector of Vasiliev recovers only light reflected along the primary axis of reflection. Light which is scattered (i.e., not reflected along the primary axis of reflection) is not recovered in the photo-detector of Vasiliev et al.
Accordingly, it is an object of the present invention to provide an improved optical detector arrangement.
It is a further object of the present invention to provide an optical detector with a significantly decreased reflection loss.